The air is alive with perhaps a dozen sweet scents at Kahuna Vapor in Ellicott City, customers adding to the aroma with every vaporous exhalation.

They're not smoking. They're "vaping" — using a battery-powered electronic cigarette that heats flavored liquid nicotine into a vapor users can inhale.

Such stores are popping up fast nationwide, quadrupling in the last year alone to about 3,000, according to an estimate by the Smoke Free Alternatives Trade Association. Kahuna Vapor, one of at least three to open locally in the last two months, opened a storefront soon after starting as an online business making local deliveries.

"We were meeting people nonstop — as soon as we were doing one delivery, I'd get another phone call," said Shawn Bowser, Kahuna Vapor's co-owner. "It just became overwhelming. We had to get a shop."

The fast-growing customer base is fueled, advocates say, by smokers who want to quit and see vaping as a less lethal way to get a nicotine fix. But the public health community is alight with debate about the products. Are they safe? Do they help smokers quit or just prolong the problem? Are the industry's celebrity advertisements and candy flavors sucking kids in?

The stakes are high. Wells Fargo analyst Bonnie Herzog predicts that e-cigarette sales will overtake conventional cigarettes in less than a decade, even accounting for the likelihood that the U.S. Food and Drug Administration will impose long-delayed rules on the now unregulated industry.

U.S. tobacco companies were slow to get in, but all of the big three are active now. E-cigarettes, Herzog wrote in a research note in November, "will be a game changer."

Pamela I. Clark, a University of Maryland tobacco researcher, is deeply conflicted about that.

On the one hand, "there's no doubt that smoking is much, much more dangerous than using an e-cigarette — they're not even in the same ballpark," she said.

But e-cigarette sales aren't restricted to smokers. And only about half the states, Maryland included, ban sales to minors. Clark worries that yet another generation will get hooked on highly addictive nicotine thanks to e-cig ads turning the idea of inhaling it "into something that's glamorous again."

Some medical professionals are convinced e-cigarettes are a net positive. Former U.S. Surgeon General Richard Carmona joined the board of a major e-cigarette maker in March, hailing the products' "harm reduction potential."

But a number of organizations — from the American Lung Association to the state health department — are not fans. Critics and those on the fence point out that filtered cigarettes were once considered a safer alternative, too.

No one knows the long-term effects of inhaling e-cigarette ingredients, including food additives and propylene glycol, that are usually eaten, said Dr. Peter Shields, deputy director of the Ohio State University Comprehensive Cancer Center.

A lot of his lung-cancer patients "swear by" e-cigarettes. He'd love to have a new cessation aid, but he's not yet comfortable recommending the devices.

"I don't think people should be out there saying, 'These things are the greatest thing, why can't we let people use them indoors, why can't physicians endorse them for their patients that are smoking,' " Shields said. "I mean, I wouldn't give someone an antibiotic without a study."

He expects fuller information in a few years. The research so far is mixed.

A November study from the University of California, San Francisco found reason to worry in a survey of South Korean teens.

"Adolescents who tried to quit smoking are more likely to use e-cigarettes but less likely to no longer smoke, which suggests that e-cigarettes inhibit rather than promote cessation," the authors wrote.

But a study published the same month in The Lancet, funded by the Health Research Council of New Zealand, concluded that e-cigarettes work about as well as nicotine patches in helping smokers stop — both are "modestly effective."

Then there's the lack of federal manufacturing oversight. The FDA — expected to issue proposed regulations soon — said in 2009 that its analysis of two of the myriad brands found, among other things, detectable levels of tobacco-specific nitrosamines, a carcinogen.

But e-cigarette enthusiasts — "vapers" — argued that the nitrosamine levels were on par with what's in an FDA-approved nicotine patch. They worry that regulators won't make vaping products better, just more costly and harder to get.

"If their thing is, 'We can't approve this until there's proof that it's 100 percent safe, I'd say, 'Hey, people are dying while you're waiting for this proof,' " said Elaine Keller, who heads the vaper-founded Consumer Advocates for Smoke-free Alternatives Association. "It doesn't need to be 100 percent safe. It only needs to be safer than continuing to smoke."